The present invention relates to hydrometallurgical extraction of copper and other valuable metals from sulphide ores and concentrates.
The present invention relates particularly, although by no means exclusively, to hydrometallurgical extraction of copper and one or more than one of the metals zinc, gold and silver from sulphide ores and concentrates.
There are limited known hydrometallurgical options for treating copper sulphide ores and concentrates to extract copper.
The bulk of copper produced hydrometallurgically on a commercial basis is from solvent extraction and electrowinning plants based on acid heap leaching of ore or from bacterially assisted dump leaching of low grade sulphuric wastes.
However, heap leaching is not a commercial option for a number of ores, particularly those containing refractory sulphide minerals such as chalcopyrite. Heap leaching of chalcopyrite (CuFeS2) ores results in poor recoveries and low extraction rates which in turn lead to the need for large leach areas and low copper content in the pregnant liquor for the solvent extraction and electrowinning steps. The non-recovery of gold is also a deterrent in cases where there is the option to produce a concentrate. Residual copper largely prevents further processing to recover gold in the heap.
Heap leaching is also not practised for zinc recovery from sulphide ores because of the difficulty in obtaining a liquor suitable for subsequent economic recovery of the zinc.
The three hydrometallurgical processes with most potential for chalcopyrite-rich sulphides are currently the Intec process, the BHAS process, and the related Cominco (CESL) process. Successful laboratory and/or pilot testing of these three processes has been reported. The processes all rely on the presence of chloride to improve the leaching efficiency for chalcopyrite without excessive attack on pyrite and oxidation of sulphur to sulphate. The BnAS/Cominco processes use conventional solvent extraction/electrowinning technology to recover copper, whereas the Intec process uses a particular novel chloride electrowinning step which produces copper powder. The BRAS process has been operated successfully on a chalcocite (Cu2S) like feed for more than ten years. However, the Intec, BRAS, and Cominco processes have not been operated commercially for chalcopyrite ores or concentrates.
There has also been considerable development of processes based on using high pressure and temperature ( greater than 200C) oxidative acid sulphate leaching of copper concentrates. These have not been commercially successful presumably because of the high costs associated with the conversion of the sulphur present to sulphate and the difficulties with economic recovery of gold and silver present from the residue using conventional cyanide based technology.
There is also active development of sulphate-based solvent extraction/electrowinning processes based on ferric sulphate leaching, low temperature oxidative pressure leaching, and bio-leaching. These processes are largely targeting the more readily leached sulphide minerals, such as chalcocite, rather than the more difficult to leach ores, such as chalcopyrite. There are no known commercial copper operations using any of these technologies.
An object of the present invention is to provide an alternative process for economic treatment of copper containing sulphide ores and/or concentrates which also contain associated valuable metals such as zinc, gold or silver.
According to the present invention there is provided a hydrometallurgical process for extracting copper and any one or more than one of zinc, silver and gold from a sulphide ore or concentrate which includes a range of mineral species that contain copper and one or more than one of zinc, silver and gold, which process includes two or more than two leach steps using leach liquors of different composition, each leach step selectively leaching one or more than one metal from the minerals, separating a solid residue and an exit leach liquor after each leach step, leaching the solid residue produced in each leach step in a successive leach step, and recovering metal from the exit leach liquor produced in each leach step.
It is preferred that the process of the present invention includes 2 or more of the leach steps summarised below which are tailored to selectively attack minerals so that each metal is recovered through a leach step (or steps) that is suitable to the minerals and the leach steps are complementary in terms of allowing materials transfer between them without one step being detrimental to and seriously interfering with the next step. The leach steps are set out below in order of increasing aggressiveness.
1. A ferric sulphate leach step to extract copper from chalcocite (Cu2S) in the sulphide ore or concentrate. This leach enables use of simple atmospheric reactors, gives good copper extraction, and allows regeneration of the leachate using bacteria or pressure oxidation of the liquor. This leach step will also dissolve part of any zinc that is present and some of the slightly more refractory copper minerals but is not ideally suited to high zinc feed. This leach does not dissolve gold or silver which remain in the residue.
2. A pressure oxidation leach in acid sulphate media with oxygen injection to extract zinc from sphalerite (ZnS). In the sulphide ore or concentrate as a first leach step or a leach step on the solids product of the ferric sulphate leach step. This leach step was originally developed by Sherritt-Gordon. This leach can also potentially dissolve copper from primary sulphides but is not very efficient for more refractory minerals such as chalcopyrite. Copper which leaches may be removed from the liquor with solvent extraction or alternatively may be precipitated into the residue by maintaining a moderately high pH (around 4) and then re-leached using one or other of the alternative leach steps. This leach does not dissolve silver or gold.
3. A mixed chloride-sulphate leach to extract copper from copper rich concentrates containing valuable levels of refractory sulphide minerals such as chalcopyrite in the sulphide ore or concentrate as a first leach step or as a leach step on the solids product of the ferric sulphate or the pressure oxidation leach steps and not amenable to leaching in sulphate-only systems. This leach step will dissolve virtually all copper sulphide minerals and zinc from sphalerite. Silver and gold remain in the residue.
4. A halex leach as described in the Intec process to extract copper, silver and gold from solids products materials. This leach step is described as part of the Intec process where it is used to leach chalcopyrite and produce copper powder through a chloride electrowinning step. This leach will dissolve all zinc and copper minerals as well as the silver and gold.
5. An oxidative acid sulphate high pressure, high temperature leach carried out at over 200C such that much of the sulphur present is converted to sulphate and copper and zinc present are leached into the liquor. This leach step has been described in numerous publications such as those, of Dreisinger, or as part of a Sherritt-Cominco process. Although this leach is more aggressive to the sulphur and copper than even the halex leach it does not dissolve the gold or silver. In the invention proposed it is potentially used as an alternative to leach step 3 in special circumstances where there is a wish to deliberately convert significant amounts of the sulphur to sulphate.
The capability of the more aggressive leach steps summarised in items 3,4 and 5 above to dissolve more of the valuable metals does not of itself mean that it is preferable to use them as a substitute for the multi-staged approach of the present invention. These more aggressive leach steps have penalties associated with the complexity of the leach and the cost of subsequent metal recovery. Leach steps 3 and 4 both include chlorides which prevent simple direct electrowinning of zinc and requires use of solvent extraction which is not to date very efficient for zinc. Similarly the need for tonnage oxygen and the greater complexity of using pressure vessels and/or chloride resistant construction detracts from the use of these more aggressive leach steps if the copper-containing minerals present are readily leachable in a simple ferric sulphate liquor.
The present invention is described further by way of example with reference to FIG. 1 which are flow sheets of preferred embodiments of the process of the present invention.
The process illustrated in FIG. 1 combines the 4 leach steps summarised in items 1 to 4 above and is applicable to sulphide ores and concentrates that include chalcocite (Cu2S), sphalerite (ZnS), chalcopyrite (CuFeS2). gold and silver.
The feed material is initially leached in ferric sulphate leach 3 to dissolve the readily-leached copper. The time, temperature and oxidation potential of this leach are controlled such that copper is selectively extracted over zinc. Some zinc dissolution may occur leading to a build up in the circuit but this can be balanced by the soluble zinc contained in the entrained liquor fed to the subsequent leach stages with the solid residue.
The solids are separated from the liquor in a solid/liquid separation step 5 and the liquor is then transferred into an electrolyte for copper electrowinning in a solvent extraction step 7.
Ferric ions are partially converted to ferrous during the ferric sulphate leach 3 and the majority of the ferrous ions are regenerated to ferric ions and returned to the ferric sulphate leach 3 using bacteria or pressure oxidation. The leach and the regeneration can be carried out simultaneously in one pressure vessel but this has penalties in complexity of equipment and also-in possible lack of selectively of the leach.
The solid residue from the solid/liquid separation step 5 is leached in a sulphate pressure leach 9 in an acid sulphate liquor with injection of oxygen at elevated pressure and temperature (around 150 psi, 150C) to dissolve the zinc present and to give a solution suitable for purification and zinc electrowinning.
The transfer of residual sulphate and iron from the ferric sulphate leach 3 into this leach does not cause difficulties as the sulphate pressure leach is also sulphate based and is capable of precipitating the iron as jarosite and/or goethite. This leach would be as currently practiced in existing commercial plants unless there is excessive copper present in the liquor due to inadequate washing from the ferric sulphate first leach 3, or because the feed material has minerals present such as bornite which are midway in reactivity between chalcocite and chalcopyrite. Alternative approaches are to force the copper in liquor to precipitate by pushing the pH up to where the cupric ions present become much less soluble or to add a specific copper extraction step such as solvent extraction into the circuit to economically recover the copper. The final traces of copper could then be removed using the conventional cementation reaction in the zinc dust purification step.
The presence of elevated levels of copper in the zinc liquor as would occur if this leach step was used directly on a copper zinc ore can lead to some difficulties in subsequent processing. Removing this copper by solvent extraction generates acid in the zinc liquor which requires neutralisation prior to final purification and electrowinning. Although this neutralisation can be done with lime which generates gypsum there is a significant cost penalty associated. The approach of a combined leach of the copper and zinc is therefore generally not favoured for a stand alone operation. At specific locations there may be scope to overcome this problem through having available zinc oxide such as from a conventional roast-leach circuit, or a copper free zinc sulphide concentrate, for this xe2x80x9cneutralisationxe2x80x9d. In that situation there is scope to consider using this pressure leach step to extract both the readily leachable copper and the zinc.
Part of the zinc in the feed material may leach during the ferric sulphate leach 3 but this does not transfer across the copper solvent extraction step 7 and goes to the sulphate pressure leach step 9 in the liquor which accompanies the solid residue from the solid/liquid separation step 5.
In cases where the zinc is extremely reactive there may be a need to neutralise some liquor to force more zinc out and prevent it building to an unacceptably high level. An alternative is to remove the zinc from part or all of the raffinate liquor from copper solvent extraction using zinc solvent extraction and to transfer it into a conventional high zinc acid sulphate solution for electrowinning. This may require prior removal of any residual cupric ions present in the liquor being fed to zinc solvent extraction using cementation and/or ion exchange.
The product of the sulphate pressure leach 9 is transferred to a solid/liquid separation step 11.
The liquor from the solid/liquid separation step 11 is transferred successively to copper removal 27, purification, and zinc electrowin 29.
The solid residue from the solid/liquid separation step 11 contains undissolved copper present in refractory minerals such as chalcopyrite plus the precious metals silver and gold. It also contains any oxide waste materials from the original feed material, unreacted waste sulphides such as pyrite, elemental sulphur, and precipitated iron in the form of jarosite and/or goethite arising during the leach reactions.
The solid residue from the solid/liquid separation step 11 is leached in a mixed acid chloride-sulphate leach 13 to extract the copper without redissolving the iron compounds or significantly attacking any pyrite present. The chloride-sulphate leach 13 avoids converting much of the elemental sulphur to sulphate. Once again forward transfer of sulphate containing liquor is not of major concern because of the sulphate base of the mixed chloride sulphate step. Any untoward build up of sulphate can be accommodated by deliberate precipitation preferably as a jarosite or through limited amounts of gypsum using lime/limestone additions.
The product of the chloride-sulphide leach 13 is transferred to a solid/leach separation step 15.
The copper-bearing liquor from the solid/liquid separation 15 is subjected to solvent extraction 7 to transfer the copper to a sulphate liquor for conventional electrowinning. If the liquor contains significant zinc due to inefficiencies in the sulphate pressure leach 9 there may be advantages including a zinc solvent extraction step to also transfer the zinc into a sulphate liquor for electrolysis. This may require first removing all of the cupric ions from the liquor probably through solvent extraction followed by ion exchange and/or cementation with zinc. An alternative where chloride is present in the liquor is to reduce the cupric ions to cuprous and rely on the chloride ions present to allow this to be stable in the liquor. The leach would oxidise these cuprous ions to cupric and allow recovery when the liquor is recycled for use.
In unusual circumstances where the ore or concentrate contains significant levels of acid consuming minerals, and/or there is a site specific need to produce some sulphuric acid at a greater rate than the small amount which arises in the chloride sulphate leach, there may be advantages in substituting a high pressure, high temperature leach (as discussed as leach type 5) for the chloride-sulphate leach 13. The remainder of the flowsheet would remain unaltered with the copper leached still being recovered via solvent extraction and the residue proceeding to the next step 15.
The solid residue from the solid/liquid separation step 15 contains gold and silver and a small amount of undissolved copper. This residue is leached in a halex leach 17 to extract gold, silver and residual copper into solution. Small amounts of residual liquor transferred from the preceding leaches is readily managed by precipitation of the contained sulphates as gypsum using lime/limestone. Residual chloride is compatible with the halex leach solution and is not of concern.
The liquor and the residual solids are separated in a liquid/solid separation step 19 and the solids are washed and neutralised for disposal. The liquor from the solid/liquid separation step 19 is then passed through carbon columns 21 to extract gold.
There may be a need to partially treat the liquor with metallic copper or another reductant prior to entry to the carbon columns 21 to ensure the redox potential is of the right order for the gold to transfer onto the carbon.
The gold depleted liquor from the carbon columns 21 is then fed into an electrowinning cell 23 to recover the copper and silver and regenerate the halex. The cell is designed so that the halex and cupric ions generated at the anode are largely prevented from returning to the cathode. This can be achieved as is done in many industries through the use of ion selective membranes as physical barriers. An alternative approach is to design of the cell geometry in terms of inlet and outlet arrangement, the use of stirrers and baffles and the possible inclusion of porous membranes such that the electrolyte predominantly flows from the vicinity of the cathode to that of the anode without backflow or back mixing.
There are a number of alternatives for this part of the process depending upon the relative amounts of the copper and silver, and also on other impurity elements that may be present. The preferred approach is to electrowin the copper and silver directly from the liquor without prior purification. This would give an impure product and also consume more power than is the case where halex leaching is being used as the primary copper production but avoids additional processing steps. The copper product in this arrangement would need further treatment to give normal high purity copper and recover the silver. This can be done using conventional copper electrorefining by melting the powder and casting it into anodes. In special cases there may be advantages in removing the silver from the liquor prior to electrowinning and purifying the liquor as proposed in the published Intec process to give a higher purity copper powder product for direct sale. This may also be beneficial if there are significant levels of mercury present in the feed to ensure it reports to a solid residue rather than into the copper metal.
In some situations, there may be advantages in reacting this liquor with a small amount of reactive copper material such as scrap metal, but if these were unavailable even a reactive primary sulphide such as chalcocite, to take advantage of the residual oxidative power and convert the majority of the cupric ions present to cuprous prior to electrowinning.
The flowsheet of FIG. 1 covers the case where the feed material contains both of the most common copper minerals plus economically significant levels of zinc, silver and gold. This is not always the case and the process can readily be modified based around the criteria of including the leach steps most suitable for the specific minerals present.